This invention relates to seals for use with well drilling and completion equipment, and more particularly to packoffs for providing metal-to-metal seals between a subsea wellhead housing and a casing hanger.
In the oil and gas industry, and especially in subsea or other underwater well drilling procedures, it is well established practice to employ an annular seal assembly, referred to as a packoff, between adjacent concentric wellhead elements, such as the wellhead housing and casing hangers that support the casing strings in the well, to pressure seal the annuli between these elements. For many years these packoffs have included elastomeric or other non-metallic annular seal elements that, when energized into tight contact with the opposed wellhead and hanger surfaces, provided the requisite pressure barrier. However, the increasing trend towards drilling deep wells into relatively high pressure strata, and the frequency of encountering hydrogen sulfide or other corrosive gases in these wells, has led to development of packoffs with all metal seal elements to establish a metal-to-metal pressure barrier. Although some of the known packoffs with metal-to-metal seals function satisfactorily under certain conditions, there is a growing industry need for such packoffs that can be installed from a remote location without difficulty, that will withstand higher operating pressure and higher corrosive environments than heretofore experienced, and that will maintain the seal throughout wide fluctuations in pressure.